The invention relates to a mass spectrometer with an electrostatic ion trap. Various types of mass spectrometers can be used for mass spectrometric analysis. Known types of mass spectrometers include ion trap mass spectrometers, such as ion cyclotron resonance mass spectrometers (ICR-MS) and electrostatic ion traps.
All mass spectrometers, however, can only determine the mass to charge ratio of an ion. Below, the term “mass of an ion” or “ion mass” therefore refers to the ratio of the mass m to the number of elementary charges z of the ion, i.e. the mass-to-elementary charge ratio m/z. Among other criteria, the quality of a mass spectrometer is primarily determined by its mass resolution and mass accuracy. The term “mass accuracy” here relates to both the statistical variation around a measured mean value and the systematic deviation of the measured mean value from the true value.
In an ion cyclotron resonance mass spectrometer, ions are confined radially by a strong, homogenous magnetic field and axially by an electric field. The strong magnetic field forces ions onto circular trajectories perpendicular to the magnetic field, where they cycle with so-called ion cyclotron frequency. The ion cyclotron frequency is proportional to the strength of the magnetic field and inversely proportional to the ion mass. By applying a high frequency electric voltage to appropriate electrodes, ions of a certain mass can be excited and move as a coherent ion package on a spiral trajectory with increasing radius. The trajectory radius of an ion package is increased until the ions pass close to detector plates so that they induce image charges on these detector plates. The image charges or, respectively, induced voltages are detected as a function of time; the time signal contains information on ion cyclotron frequencies of all orbiting ion packages and number of ions in the respective ion packages. A Fourier transformation can be used to transform the measured time signal into a frequency spectrum, which can be converted into a mass spectrum via the known mass dependence of the ion cyclotron frequency. With an ion cyclotron resonance mass spectrometer, a very high mass resolution of more than 1,000,000 can be achieved for measurement times of about one second. However, the manufacture and operation of an ion cyclotron resonance mass spectrometer with high mass resolution are quite costly because only superconducting magnets with helium cooling are able to generate magnetic fields with necessary strength of approximately 10 Tesla.
From an early publication by Kingdon (Physical Review, 21, 1923, p. 408-418: “A method for the neutralization of electron space charge by positive ionization at very low gas pressures”) it has already been known that ions can be stored in an electrostatic field if they move with sufficient kinetic energy around an electrode with an ion-attracting electric potential. In U.S. Pat. No. 5,886,346 A, another electrostatic ion trap is disclosed, which has been marketed under the name Orbitrap™. The Orbitrap™ consists of a single, spindle-shaped, inner electrode and a coaxial outer electrode, wherein an ion-repelling electric potential is applied to the outer electrode and an ion-attracting electric potential is applied to the inner electrode. With the aid of an ion-focusing system, ions are injected as ion packages tangentially to the inner electrode, and move between the inner and outer electrode in a hyperlogarithmic electric potential. The kinetic injection energy of the ions is selected such that attracting forces inside the electrostatic ion trap and centrifugal forces balance each other. The injected ions therefore move on circular trajectories. In the direction along the trap axis, the Orbitrap™ comprises a harmonic potential. The oscillation frequency of ions in a harmonic potential is inversely proportional to the square root of their mass. Like in an ion cyclotron resonance mass spectrometer, ion packages oscillating inside the electrostatic ion trap induce image charges and corresponding voltages which are measured as a function of time. The mass resolution of an Orbitrap™ is currently around 100,000. The advantage of the Orbitrap™ compared to an ion cyclotron resonance mass spectrometer is that it does not require a superconducting magnet, and thus the device-related complexity and costs for operation are less. However, the mechanical specifications concerning the electrode system are very high. Moreover, the injection of ions is quite critical because ions of different masses have to be injected at nearly the same time in order to be stored in the Orbitrap™. A further challenge consists in the requirement that the kinetic energy of ions to be injected must vary only within a small range.